The state's northern beaches dodged significant damage from this week's storm in part because of lucky timing.

Waves off Coffs Harbour peaked at almost 12.5 metres late on Monday, with significant wave heights averaging about seven meters. “That's about a one-in-ten or one-in-11 year event,” Phil Watson, Principal Coastal Specialist in the Office of Environment and Heritage, said.

Waves might have been higher still off Byron Bay – had the storm not carried off the Waverider buoy, one of seven operated off the state's coast by the government.

“We're not sure what happened to that one,” Dr Watson said. “That would have been a wonderful record there.”

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Fortunately for the region, the storm, a deep low-pressure system generated by former tropical cyclone Oswald, came midway between king tides.

“We're not hearing at this stage of reports of any damage to any of the structures or crumbling of structures or damage to buildings,” Dr Watson said.

“Had it been a fortnight ago on the peak of the king tide, then we would have had far more damage and coastal erosion,” he said. “Water levels a couple of weeks ago were in the order of 20 centimetres higher.”

Beaches of particular concern, such as Lake Cathie, just south of Port Macquarie, Brooms Head, Wooli, and Byron Bay “look like they've survived intact”, he said.

The timing was also fortunate because most of the state's beaches went into the wild weather with a significant buffer of sand that had been built up in recent months. That left them at risk, however, if another big system moves through before they are naturally replenished.

Each of the Waverider buoys are anchored about 8-12 kilometres offshore and weigh about 200 kilograms. One or two of the $40,000 devices break from their moorings each year, with several of them retrieved from as far away as Cape York or New Zealand.

'Horrendous' surf

Elsewhere along the coast, wave buoys at the Gold Coast and Tweed River recorded a maximum wave height of 15 metres on Monday, said Sam Brown, a meteorologist at Weatherzone.

Sydney also saw large waves, reaching as high as 9.6 metres early on Tuesday.

"Conditions have been quite horrendous in the water though, with all the rain over the past few days increasing run-off, making for dirty brown and murky water," Mr Brown said. "Gusty winds are also making it very bumpy and choppy at the beaches with messy seven-to-10 foot waves."

To the south of the state, Batemans Bay saw wave heights reach six to eight metres on Tuesday, he said.

Conditions in the water and near the coast are likely to be treacherous.